Strengthening in MULTIPHASE (MP35N) alloy: Part I. ambient temperature deformation and recrystallization
- PDF / 4,053,167 Bytes
- 13 Pages / 613 x 788.28 pts Page_size
- 88 Downloads / 167 Views
I.
INTRODUCTION
M U L T I P H A S E , * MP, alloys are a family of cobalt*MULTIPHASE is a trademark of SPS Technologies, Inc., Newtown, PA.
based alloys whose first member was developed by Smith. t~J This original alloy, known as MP35N, contains 35 pct Co, 35 pct Ni, 10 pct Mo, and 20 pct Cr. It is characterized by a remarkable combination of ultrahigh strength, high ductility, and a high resistance to fracture combined with high corrosion resistance. The later alloys, MP159 and MP210 tz,3,aj (Table I), were empirically developed to give strength retention to higher temperatures. These alloys, despite their very high materials cost, have such valuable properties that they are widely used for critical applications, such as aerospace fasteners and for extreme conditions in oil extraction. The alloys were initially designed on the basis of the fcc (a) ~ hcp (e) (fcc, face-centered cubic; hcp, hexagonal close-packed) martensite phase transition that occurs on cooling pure cobalt below 420 ~ Addition of nickel stabilizes the fcc phase and depresses the martensite start temperature, Ms, so that at 35 wt pct Ni, the Ms falls below ambient
RISHI PAL SINGH, Senior Engineer, is with Westinghouse Electric Corporation, Orlando, FL 32826. ROGER D. DOHERTY, Professor, is with the Department of Materials Engineering, Drexel University, Philadelphia, PA 19104. Manuscript submitted October 30, 1989. METALLURGICAL TRANSACTIONS A
temperature, t61 The molybdenum and chromium additions have the opposite effect: They both raise the Ms in the binary Co-Mo and Co-Cr alloys, tTj and Mo is known to stabilize the hcp phase in the Co-Mo binary system.t8] The physical-mechanical metallurgy of the MP alloys is very interesting but little understood. I9-131 Since the later alloys, MP159 and MP210, are chemically more complex than the simpler MP35N, the investigation into the metallurgy of the alloy system in this and Part II t~41 will mainly use MP35N to try to establish the operating mechanisms. A later article ]~Sj will explore the alloys in which an additional strengthening process (precipitation hardening) has been demonstrated, tl6,171 The alloy MP35N has been shown to be capable of forming significant amounts of only the e phase from the a matrix. Despite the presence of e-stabilizing Mo and Cr, MP35N has not been seen to form thermal martensite on cooling to ambient temperatures or even on refrigeration to 77 K. tl61 However, with room-temperature deformation, very strong work hardening is seen (Table I), with the formation of a very high density of platelike features on the a {111} planes. These plates were initially assumed to be just e martensite, c~,9-11~ The hardening mechanism was expected to be strain- (or stress-) induced martensite, as in metastable austenitic steels, t~8,~9,2~ especially the transformation-induced plasticity (TRIP) steels, t2~,221 Transformation-induced plasticity steels are, of course, well known for having very high strengths, combined with both high tensile ductilities and very high toughnesses.t2z] VOLU
Data Loading...
